At the present time, the most accurate measuring techniques in portable instrumentation are variations of the Vernier technique. This perhaps reaches its greatest development in the marine sextant, wherein a gear sector is traversed by a worm gear on an arm pivoted at the center of the sector. The angular position of the worm gear is read out by a Vernier. The maximum readout is in the order of 0.02 arc minutes while the overall system accuracy at room temperature is in the order of 18 arc seconds due to the eccentricity of the arc. Such a gear sector typifies the limits of gear hobbing technology and constitutes a heavy, expensive, delicate scale for analog readout of shaft position (in the case of the sextant, of the elevation of the index mirror).
The goal which led to the development of the present instrument was to derive a means for reading out the index mirror limitation by an inexpensive, mass-producible, rugged lightweight, electro-optical means suitable for replacing the sector gear with a readout in electrical signals, suitable for electronic display and computer-compatible.
Commercial shaft encoders are operated by Moire technique, and are limited by defraction to something of the order of 100 line pairs per millimeter. Additionally, these are intended to measure 160 degrees and greater, whereas for geodetic measurements, horizon-to-zenith measurement is required. To achieve a design goal of .+-.3 arc sec. revolutions (6 arc seconds between counts) with quadrature, some 64,000 line pairs are required. This implies the use of a Moire encoder sector of some 25 inches in radius.
The sector encoder for geodetric instruments herein disclosed has five important advantages:
1. The informational and reference gratings are made flat, which lends itself to fabrication by conventional linear fabrication techniques. It is made on a flexible substrate such that it may be bent around and affixed to an arcuate cylindrical mandrel. Cylinders of revolution may be fabricated to extremely close tolerance by ordinary machine shop practices; so that the resultant assembly yields a cylindrical scale which can be accurately divided into equal angular measurements, while bypassing the extremely complex fabrication of dividing an arcuate scale in place.
The substrate of the scale is matched to the sector mandrel in thermal expansion, such that the scale varies only in radius over temperature extremes, maintaining its angular fidelity. In a reflective embodiment, the scale substrate and mandrel may be made of, for example, aluminum. In a transmissive embodiment, the scale substrate may be thin glass sheet (such as Corning Co.'s "Microsheet") on a stearite, glass, or alumina sector.
2. While avoiding the cost and weight of a hobbed gear sector, an increase in accuracy is obtained. The 6 inch radius gear sector is capable in production of maintaining a readout accuracy within 18 arc seconds. Assuming 50,000 counts per inch (as described below), the optical readout device (due to its linear perfection in the flat, the extreme circulatory, conventionally readily available with lathe machining, and the assembly techniques described below), utilizing the same radius would have a readout accuracy within 1.5 arc seconds (.+-.0.75 arc seconds.
3. The readout, summed in an electronic computation stage, directly yields a computer-compatible signal for further processing, reading the sector rotation from some assigned "null" position.
4. The readout utilizes an easily mass-produced linear grating, an inexpensive optical collimetric chain, and a low-cost light-emitting diode and detector pair. Utilization of a reference grating rather than a single slot or pinhole in the input beam permits use of an LED rather than an expensive and cumbersome gas or diode laser. As well, the grating may be divided into two sections, 90 degrees out of phase in spatial frequency to yield a quadrature signal, with 4.times. to 12.times. multiplication of sensitivity.
5. Finally, the entire readout mechanism uses only low voltage (1.5-3 volts) at moderate currents (25 to 100 ma), making it eminently suitable for hand-held, small, battery-powered instrumentation.